1. Field of the Invention
The present invention relates to a process for the preparation of 4-aminobut-2-enolides starting from 4-alkoxyfuran-2(5H)-one or 4-arylalkoxyfuran-2(5H)-one.
2. Description of Related Art
Certain substituted 4-aminobut-2-enolide compounds are known as insecticidally effective compounds from EP-A-0 539 588 and WO 2007/115644. They can be synthesized by various methods.
Thus, for example, Heterocycles Vol. 27, No. 8, pages 1907 to 1923 (1988) and EP-A-0 539 588 describe that enaminocarbonyl compounds (3) can be prepared from anhydrous tetronic acid (1) and an amine (2), as shown in Scheme 1. However, this process is not very suitable for the large-scale industrial production of enaminocarbonyl compounds since the anhydrous tetronic acid (1) cannot be produced in a cost-effective manner.

To date, tetronic acid has not been commercially available in relatively large amounts, meaning that it has to be prepared starting from acetoacetic ester via bromination and subsequent hydrogenation (cf. Synthetic Communication, 11(5), pages 385 to 390 (1981)) for use in the process described above. The relatively low yield of tetronic acid (usually less than 40%) and the condition that tetronic acid must be anhydrous gives rise to high costs.
A further process for the preparation of tetronic acid is described in Swiss Patent Specification 503 722. 4-Chloroacetoacetic ester is reacted with an aromatic amine to give 3-arylaminocrotono-lactone, the tetronic acid being liberated following subsequent treatment with mineral acid. The tetronic acid can be isolated only by distillation under a high vacuum, which is disadvantageous for the large-scale industrial use of this process.
EP-A-0 153 615 likewise describes a multi-stage process for the preparation of tetronic acid which starts from 2,4-dichloroacetoacetic esters and which is not very suitable for large-scale industrial production. This process requires many complex stages and produces the desired tetronic acid in a comparatively moderate yield of 65%.
A further process for the preparation of tetronic acid starting from malonic esters and chloroacetyl chloride is known from J. Chem. Soc. Perkin Trans. 1 (1972), 9/10, pages 1225-1231. This process produces the desired target compound, but with a yield of only 43%.
Tetrahedron Letters, No. 31, pages 2683 and 2684 (1974) describes inter alia the preparation of tetronic acid, which is shown in Scheme 2. The starting material used here is dimethyl acetylenedicarboxylate.

A disadvantage of this process is the low overall yield of only 30%, and the use of expensive starting materials, such as, for example, lithium aluminium hydride (LiA1H4).
WO 2007/115644 describes the preparation of specific 4-aminobut-2-enolides. For example of 4-[[(6-chloropyridin-3-yl)methyl](3,3-dichloroprop-2-en-l-yl)amino]furan-2(5H)-one by reacting 4-[[(6-chloropyridin-3-yl)methyl] amino] furan-2 (5 H)-one with 3-bromo -1,1-dichloroprop -1-ene or by reacting 4-[[2-fluoroethyl)amino]furan-2(5H)-one with 2-chloro-5-chloromethylpyridine. The reactions are preferably carried out with hydrides of lithium or sodium. Here too, these substrates are expensive and can only be handled with difficulty for safety reasons.
A further process for the preparation of substituted 4-aminobut-2-enolides is described by Mowafak. et al. in J. Heterocyclic Chem., 21, pages 1753-1754 (1984). This process starts from methyl tetronate, the desired compounds being prepared as a result of reaction with amines. However, this described exchange of the methoxy group described therein via a “Michael Addition” can only be carried out with electron-rich cyclic or primary amines, which is demonstrated by the example in the experimental section of this patent application.